This is a critical engineering consideration known as the "thickness effect" or "size effect." The mechanical properties of Q355B steel degrade with increasing thickness, particularly in terms of yield strength, tensile strength, and impact toughness. This is explicitly accounted for in standards like GB/T 1591-2018.
Here's a detailed breakdown of how and why properties change, and the key data:
1. Key Property Changes with Thickness
a) Yield Strength (ReH) and Tensile Strength (Rm) – STRENGTH DROPS
As plate/thickness increases, the guaranteed minimum yield and tensile strength decrease. This is the most important design factor.
Standard Values from GB/T 1591-2018 for Q355B:
| Product Thickness (t) or Diameter (mm) | Yield Strength (ReH) Min (MPa) | Tensile Strength (Rm) (MPa) |
|---|---|---|
| t ≤ 16 | 355 | 470 - 630 |
| 16 < t ≤ 40 | 345 | 470 - 630 |
| 40 < t ≤ 63 | 335 | 470 - 630 |
| 63 < t ≤ 80 | 325 | 470 - 630 |
| 80 < t ≤ 100 | 315 | 470 - 630 |
| 100 < t ≤ 150 | 305 | 450 - 600 |
| 150 < t ≤ 250 | 285 | 450 - 600 |
| t > 250 | 265 | 440 - 590 |
Example: A 20mm thick plate has a min yield of 345 MPa, while a 100mm thick plate has only 315 MPa-a ~11% reduction. Designers must use the correct value for their specific thickness.
b) Impact Toughness – TOUGHNESS DROPS
The guaranteed Charpy impact energy at +20°C (≥34 J for Q355B) applies to standard 10x10mm specimens. For thicker plates:
Through-Thickness Inhomogeneity: The core of a thick plate cools slower during rolling, leading to a coarser grain structure and lower toughness compared to the finer-grained surface.
Specimen Location: If impact specimens are taken from the 1/4 thickness or core position (as required for heavy plates per some specs), the measured values will be lower than those from a surface location.
Practical Implication: For critical applications (e.g., seismic, offshore), thick plates of Q355B may require upgrading to Q355C/D/E to guarantee adequate through-thickness toughness.
c) Ductility – SLIGHTLY REDUCED
Elongation (A%) also has a slight thickness dependency in the standard, with minimum requirements decreasing modestly for thicker sections (e.g., from ≥21% for t≤40mm to ≥19% for t>63mm).
d) Through-Thickness Properties (Z-Direction) – SIGNIFICANTLY REDUCED
Lamination Risk: Thicker plates are more prone to segregation of impurities (S, P) at the mid-thickness during solidification.
Z-Direction Tensile Strength: If the design requires through-thickness strength (to resist lamellar tearing at welded joints), standard Q355B may be insufficient. Z15, Z25, or Z35 quality steel (per GB/T 5313) with guaranteed reduction of area in the thickness direction must be specified for thick, highly restrained welded connections.
2. Root Causes of the Thickness Effect
Manufacturing Process (Rolling & Cooling):
Surface Quenching Effect: Thinner plates cool faster after the final rolling pass, resulting in a finer ferrite-pearlite microstructure at the surface, which is stronger and tougher.
Slower Core Cooling: The core of thick plates cools much slower, allowing grains to grow larger, resulting in a coarser, weaker microstructure.
Less Rolling Reduction: The core of a thick slab undergoes less mechanical deformation (reduction) during rolling compared to the surface layers, which also contributes to coarser grains.
Statistical & Metallurgical Factors:
Probability of Defects: Thicker sections have a larger volume of material, increasing the probability of containing inclusions, micro-voids, or chemical segregation that can weaken the material.
Residual Stresses: Higher residual stresses from uneven cooling in thick plates can effectively lower the measured yield strength.
3. Critical Implications for Design & Fabrication
| Activity | Implication of Thickness Effect |
|---|---|
| Structural Design | The design strength (fy) MUST be selected based on the actual member thickness. Using the "t ≤ 16mm" value (355 MPa) for a 50mm thick beam is non-conservative and unsafe. Always refer to the thickness-property table in GB/T 1591. |
| Material Procurement | In purchase orders, specify both the grade (Q355B) and the thickness range (e.g., 40 < t ≤ 63 mm) to ensure the mill supplies steel meeting the correct strength guarantee. |
| Welding of Thick Plates | Preheat and controlled heat input become mandatory to prevent excessive hardening and cracking in the HAZ of thick Q355B sections. Consider buttering or using transition joints for very thick welds. |
| Critical Applications | For thick plates in seismic moment frames or offshore nodes, consider upgrading: • To Q355C/D/E for better toughness. • To Q355GJ (high-performance steel) for more stable strength across thicknesses. • Specifying Z-directional properties to prevent lamellar tearing. |
Summary
The mechanical properties of Q355B steel are not constant; they are a function of product thickness. With increasing thickness:
Yield and tensile strength decrease (per standard tables).
Impact toughness, especially at the core, decreases.
Risk of internal defects (laminations) increases.
Therefore, engineers must never use a single "handbook value" for Q355B. The thickness-specific minimum values from the material standard (GB/T 1591-2018, Table 2) are the legally binding values for design and construction. For critical thick-section applications, supplementary requirements for toughness and through-thickness properties are often necessary.